Braking system for three phase motors

Abstract

A braking system for three phase motors without an accessible neutral terminal comprising a current limiting motion detection circuit that is energized when the motor is energized or is being supplied with braking current as long as the rotor is in motion, a DC braking circuit, and a control circuit actuated by a stop command that disconnects the source from the motor and supplies braking current until the rotor comes to a stop. A ferroresonant motion detection circuit is disclosed that is also useful in braking systems for other motor types.

Having thus described my invention, what I claim is: 
     
       1. A braking system for three phase motors having at least first, second and third accessible terminals, comprising a pair of impedances connected in series across said first and second terminals, voltage sensing circuit means connected between said third terminal and the junction of said impedances for producing a first signal in responce to a voltage therebetween, and means responsive to an applied signal in the presence of said first signal for supplying direct current to a pair of said first and second terminals. 
     
     
       2. A motion detection circuit for a three phase induction motor having first, second and third current supply terminals, comprising first and second capacitors connected in series between said first and second terminals, inductive circuit means connected between said third terminal and the junction of said capacitors, and means responsive to a voltage above a predetermined threshold value applied to said inductive circuit means for producing an output signal, in which said inductive circuit means is ferroresonant with the parallel combination of said capacitors at a voltage near said threshold value. 
     
     
       3. A braking system kit for a polyphase induction motor adapted for wiring at a predetermined frequency and at a first voltage, said kit comprising a pair of capacitors, a pair of resistors, a rectifier, a motor operation sensing circuit adapted to be connected to said motor, and control circuit means adapted to be connected to supply current from an alternating source to said motor through said rectifier as long as motor operation is sensed by said operation sensing circuit, whereby the motor can be adapted for braking at said first voltage by connecting said resistors in parallel with said capacitors and said capacitors in series with said source and said rectifier, and the motor can be adapted for braking at said second voltage by connecting said resistors and said capacitors all in parallel and in series with said source and said rectifier. 
     
     
       4. In a braking system for induction motors having a rotor and a stator, at least first and second windings adapted to have a voltage induced across said first windings when pulsating direct braking current is supplied to said second winding and when and only when said rotor is in motion with respect to said stator, a rotor motion sensing circuit comprising a series combination of inductive and capacitive reactors in ferroresonance at a point near the point of zero relative motion between said rotor and said stator, and motion detecting means responsive to the voltage across said inductive reactor, for producing a first signal or a second signal according as the voltage across said inductive reactor in above or below a predetermined value, respectively. 
     
     
       5. A braking system for a polyphase induction motor having a rotor and a stator and at least first, second and third accessible terminals adapted to be connected to an alternating current source, comprising, in combination, first switching means operable from a first state to a second state, second switching means operable from a first state to a second state, first circuit means controlled by said first switching means in its second state and said second switching means in its first state for supplying operating current from said source to said terminals, third switching means operable from a first state to a second state, second circuit means controlled by said first switching means in its first state, said second switching means in its second state, and said third switching means in its first state for supplying pulsating direct current to at least said first terminals, and ferroresonant circuit means connected to at least a pair of said terminals and responsive to pulsating direct current supplied to said first and second terminals for operating said third switching means to its second state when the voltage across said pair of terminals falls below a predetermined value. 
     
     
       6. A braking circuit for three phase motors having at least first, second and third accessible terminals, comprising a pair of capacitors connected in series between said first and said second terminals, an inductive circuit element connected between said third terminal and the junction of said capacitors, and means controlled by the voltage across said inductive elements and by an applied signal for supplying pulsating direct braking current to at least two of said terminals so long as the voltage across said inductive element is above a predetermined value. 
     
     
       7. The apparatus of claim 6, in which said inductive element is ferroresonant with said capacitors. 
     
     
       8. A braking system for a polyphase motor having a rotor and a stator and a plurality of windings at least a first of which is excited with alternating current when pulsating direct braking current is supplied to at least one of said windings and said rotor is in motion with respect to said stator, comprising ferroresonant circuit means connected to said first winding for producing a first signal or a second signal according as said first winding is or is not excited, and means operable to apply pulsating direct braking current to at least said one of said windings when and only when said first signal is present.